Cross-linking of fluorinated elastomers



United States Patent cRoSS-LINKING on FLUORINATED I ELASTOMERS Francis J;- Honn, Westiield, and Willard.M.'Sims, Hackensack, N. 1., assignors, by mesne assignments, to Minnesot'a Mining and ManufacturingCompany, St. Paul, Mmm, a'corporationof' Delaware No-Drawing. ApplicationJune' 14, 1954 Serial No. 436,714

1 8 Claims. c1. 260-87.7)

This invention relates to the cross-linking of polymers and, more particularly, to the modification of the characteristics'of chain saturated polymers whose structure is composed substantially exclusivelyof carbon, hydrogen and halogen atoms. This applicaion is acontinuation-inpart of application Serial No. 346,800, filed April 3, 1953.

Chain polymers are thermoplastic in nature. They exhibit continuous flow under the influence of. heat and pressure. Such polymers can be resoftenedas. often as desired and are usually soluble in selected solvents.

Cross-linked or space polymers,- on the other. hand, are generally thermosetting, that is to say that they cannot be softened without decomposition once they have hardcried, and that they are insoluble inallsolvents. A chain. polymer may, however, containa small number of crosslin'kages without completely losing its thermoplastic properties.

It is'often' desirable to convert chain to space p'olymers.

This is done when it is desired todecreasesolubility, and.

thermoplastic flow and, in the case ot chain elastomers, where it is desired to obtaina harder, tougher-product. The cross-linking of elastomers is commonly referred to as vulcanization. I-

Vul'canized or cross-linked elastom'ers mayhave equal, greater or lesser extensibility than the linearelastomer's, depending on the numberand nature 'of'the cross-linkages. In any case, they have. lessened plasticity and solubility and increased toughness and heat resistance; Q

The nature. of the cross-linking. agent will ya'ry with the natureot the chain polymer and with the nature of the'cross-link desired. There are a number of reactions which may result in the cross-linking ofia chain polymer. In some reactions the cross-linking agent will take part so that at least a portion of it forms the crossdink and appears in the final cross-linked molecule. In other reac'- tions the cross-linking agent acts solely asan activator and does not appear in the final molecule.

The chemistry of cross-linking is imperfectly understood. Cross-linking agents successful with one type of chain polymer are often inelfective with another type. There appears to be no rule for predicting with accuracy whether ornot' a particular cross-linking agent etfe'ctive.

with one type of chain polymer will be 1efiective with another In fact, the term fcrosslinkingagent, itself, covers material's whicharechemically and physi; cally dissimilar and have in common the sole characteristicthat*each is etfective for the 'crossdinking, ofat least one chain polymer. i I

Among the mos't'useful of the thermoplastic resins are.

' those prepared bythe polymerization of highly halogenated 'mon'oolefi'nsand particularly those prepared from highly fiuorinated ethylenes. It is to be understood that metering fhalo g'en'atedff and ffluorinated. as used herein n ed M r 9 2 are intended to designate halogen or fluorinesubstitution. on a material regardless of. whether or not. such substitution is obtained by direct halogenation or fluorination of the'rnaterial so designated. These, resins,, particularly. when-they are at least half fiuorinated, i. e-., containing; fillOl'lIlfiSllbStltllfiOfl at at least half of the positions avail able for such substitution, have. great chemical stability of elasticity, depending on the degree of disorder. Disorder in a chain polymer is often obtained by the .copolymerization of mono-olefins, and. for purposes of this in vention,.itis necessary that at least one of the co-monomer olefi-ns should contain hydrogen. 7

For example, chlorotrifluoroethylene polymers. have been developed. to. commercial succes due. to their unique combination. of physical, chemical, and electrical prop.

erties It has been, found possible to. modify some; of

these properties for greater utility in specific applications. ,by the copolymerization of chlorotrifiuoroethylene with hydrogen containing, mono-olefins, such as vinylidene, fluoride. These copolymers still retain many of the. valuable properties of chlorotrifluoroethylene homopolymers, and, particularly retain, to a large degree, the property of chemical'inertness. H p p 7 It hasbeen. foundfthat copolymers of chlorotrifllloro ethylene and vinylid'ene' fluoride. containing up "to about 5 mole percent of vinylidenefiuoride are more thermoplastic than the homopolymer ofchlorotrifluoroethylene and can, therefore, be molded easily into thin shapes which are diflicult to mold with the homopolymer. Copolymers of chlorotrifluoroethylene and between 20 and 30 mole percent of vinylidene fluoride are soluble at ambient temperatures in oxygenated organic solvents, such as. esters, ketones and, cyclic ethers, and may be applied as coatings from such solutions It has also been found that copolymers of chlorotrifluoroethyleneand vi-nylidene fluoride containing from about .30 to-8O percent of. vinylidene fluoride are elastomericin nature andretain their elastomeric. properties over a-widc temperature range. All of these copolymers are chemically inert, 10 a large degree.

It is an object of this invention to cross-link highly heatstability; but adequatezor even superior exteusibility.

:It is a turther object-of this invention to producehard thermosetting vulcanizates ot highly fiuorinated, linean.

saturated, hydrogen-containing elastomers.

"It is, a further objectxo'fthis invention torcducevthe: solubility of :highly fiuorinated, linear; 1saturated,hyclro=: i gen-containing elastomers" produced by the copolymeriz a tion of at least two mono-olefinic compounds, angles-st one of whichishighly halogenated ana' 'a't 1east one of:

which-cont h ro -'1 Itjis a f urtherobject ofjthisinveutiont V inertcoatings from solutions of highly fluorinated, saturated, linear, hydrogen-containing elastomers. It is a further object of this invention to produce tough, chemically inert, self-supporting films from solutions of highly fluorinated, saturated, linear, hydrogen-containin elastomers.

Other objects will appear' hereinafter. These and other objects are accomplished by the following inventionz Y Highly halogenated chainpolymers whose structure is composed substantially exclusively" of carbon, hydrogen and halogen atoms, are reacted at elevated temperatures withan organic peroxy compound which is stable against decomposition below about 50 C., and with a basic metal compound, to produce a cross-linked polymer. It is preferred that the chain polymers be at least half fluorinated and that CH groups be present. In other words, at least some of the carbon atoms on the chain should be bonded only to hydrogen atoms and other carbon atoms.

Chainpolymers with a minor degree of disorder in their 'molecules are generally more thermoplastic in nature than those without such disorder. Elasticity requires a greater degree of disorder. In order to obtain an elastomeric product, at least percent of'the carbon atoms in the structure must be of the type which are bonded only to hydrogen atoms and other carbon atoms. Ordinarily, a mono-olefinic compound containing a CH group is used to provide these carbon atoms, and this results in chains containing CH,- groups. Ordinarily, such a monoolefinic compound is copolymerized with another highly fluorinated mono-olefin to help give the polymer molecule. its highly fluorinated nature.

Among the highly fluorinated mono-olefins which may be used as comonomers are; CF,=CFCI, CF =CCl Among the hydrogen containing mono-olefins which maybe used as comonomers with the above highly fluorinated mono-olefins are: CFg=CH2, CFH=CH Of these, the most advantageous combinations are:

The aforementioned mono-olefins may be copolymeriud in any proportions, depending on the characteristics desired. Good chemical stability requires fluorine substitution at at least one-half of the possible positions for such substitution in the polymer chain.

It is to be noted that all ofthe monomers enumerated above contain only. carbon, hydrogen and halogen atoms and, therefore, produce. copolymers substantially without markedly reactive functional groups. This, of course, does not preclude the presence of other atoms in end groups which might be formed by chain stoppers in the polymerization zone, present by design or by accident. Such end groups are relatively few in numher and are difficultto detect. They do not, in any way,

. ganic peroxy compounds also include peresters, such as pounds which are acid acceptors, such as magnesium 4 afiect the characteristics of the polymer to any noticeable degree.

The copolymerization reaction may be carried out in either a water suspension type system or in a mass polymerization system. In the former system the reaction is preferably carried out at a temperature between about 0 C. and about 35 C. In a mass polymerization system the reaction is preferably carried out at a temperature between about 20 C. and about 0 C. With the Water suspension type system a redox catalyst system is preferred. It has and contains an oxidant, a reductant and a variable valence metal salt. The oxidant in the water suspension type recipe is preferably an inorganic persulfate, such as potassium persulfate, sodium persulfate or ammonium persulfate. The reductant is preferably a bisulfite, such as sodium bisulfite or potassium bisulfite. The variable valence metal salt which is employed for the purpose of regenerating the oxidant is preferably in the form of an irong'anic peroxides suitable for carrying out the polymerization are trifluoroacetyl peroxide, difluoroacetyl peroxide, 2,4-dichlorobenzoyl peroxide, chloroacetyl peroxide, trifiuorodichloropropionyl peroxide, and dichlorofluoroacetyl peroxide.

By the method of this invention these copolymers may be cross-linked to produce from the elastomers either soft vulcanizates of increased strength and toughness, of decreased soubility, and of adequate or even increased extensibility, or hard vulcanizates which are thermosetting. From the thermoplastic resin a formed thermosetting resin may be produced.

The organic peroxy compounds used in this invention must be stable below about 50 C., or else they will cause cross-linking while they are being blended into the copolymer. Among these compounds are the acyl and acoyl peroxides and hydroperoxides, such as ditertiary butyl peroxide, dilauryl peroxide, dibenzoyl peroxide, p-chlorodibenzoyl peroxide, cyclohexanone hydro peroxide and ditertiary butyl hydroperoxide. The oralkyl and aryl perbenzoates and perphthalates, including specifically ditertiary butyl perbenzoate and ditertiary butyl diperphthalate. Peracids, such as tertiary butyl permaleic acid and perlauric acid, are also included.

Among the basic metal compounds which may be used, the preferred compounds are the basic oxides, such as magnesium oxide, calcium oxide, zinc oxide and lead oxide (PbO). Other basic metal compounds which may be used include both inorganic and organic metal comcarbonate, calcium carbonate, calcium acetate, tetraphenyl tin, tetraethyl lead, and sodium methylate.

- While it is not desired to be bound by any particular theory .of operation, it is believed that the peroxy-type compounds remove a hydrogen atom from a carbon atom on the linear chain and thereby produce an activated free radical spot on the chain. This spot links directly to a similar free radical spot on another chain, and thus produces a cross-linked polymer. It is believed that the basic metal compound helps to remove the hydrogen atom and to neutralize it after removal, and thereby facilitates the cross-linking reaction.

The cross-linking reaction may require or may produce materials which have an adverse etfect on the properties of the cross-linked polymer. For example, the metallic salts produced by the reaction of the inorganie basic .metal compounds with the acidic hydrogen removed from the polymer chain, might reduce the chem rrier for special purposes where a high degree of chemical iriertness or electrical resistance is required, conditions may be controlled so that a minimum of undesired material remains in the polymer.

There are several methods of reacting the chain .polymers with the cross-linking agents of this invention, 'depending on the character of the polymer, the character of the cross-linking agent andthe character of the desired product. Cross-linking agents may be easily incorporated into the elastomers by mechanical mixing, either with or without plasticizers. Such mechanical mixing involves shearing forces and is carried :out in equipment such as 2-roll mills, Banbury (internal) mixers and screw-type plasticators, which resemble extruders. :Somewhat .elevated:'temperatures of the order of from about 50 to .aboutTS .C.=ordinarily. prevail in the mix'ing operation nine to the :mixing action itself. Articles to be molded are then heatedin the mold with additional aheat, as by:hot air, :steam or :hot press platens, thereby shaping and cross-linking simultaneously. The temperature inrthe mold mayrangefromabout 100 C. to 200 C.

ssince anechanicalamixinggenerates'heat and since it is difficult to blend other materials .into a cross-linked polymer, {it is :usually desirable to blend other materials intotthezpolymerifirstz and *thenhadd the linking; agentsi last,

just-heforezfabrication. :Among'the other-materials which might-:hevaddeddo thezaforesaid-polymers prior to crosslinkingare*fillerszpigmentsand plasticizers.

:With:certaintpolymersandffor certainuses, particularly for coatings, polymers :may -be .cross linked "from solutions. flhe polymer and-"linking agents :are blended togethenand then :dissolved in assuitable solvent. When theirhasiclmetalcompound; is an insoluble compound, such asizincoxideditrremains suspended in' the yiscous solution. Thet solution is 'applied to :asurface, such as a fabric or a metal, and then the coating is dried and-heated to crosslinktthezpolymer. In some cases, the-coating 'is adherent andi-inr-other casea it may be stripped ofi to form a selfsupporting film o'f eross linked polymer.

Still another methodtof cross-linking a chain polymer involves the use o f a millgy emulsion or latex. As stated above, the copdlymerization product of chlorotrifluoroethylene and winyliclene fluoride maybe-prepared in a Water suspension type system. In Suchasystemthe productgis removed from the reactors asaflatex. Forlmost purposes, the a dry rubbery copolymer .is recovered .by coagulationsof the -latex with; 5 11s acids followed by washing and drying. For otherpurposes, however, such as dip coating and spraying, the latex can be used die lr- :Inathesecasestth othermingr difints (fi lerscrosslinking agents, etc )-are dispersed in .water containing a surface-active agent, and these dispersions are blended withs'theil-atex. 'The latex is themapplied as:a coatingto a surface (similar :lto zsolutions disclosed above) :and the polymer is cross-linked as the latex is dried and heated.

' -5--partsby-weight.

Non-elastomeric chain polymers may be cross-linked after molding to produce articles which do not flow at. elevated temperatures. The molding powder may be blended with the peroxy-type compound and the basic metal compound, and the mixture may then be molded as usual. After molding, the article is heated in the mold for an extended period and then may be heated after removal from the mold in an oven to complete the cure.

Still another method of reacting the chain polymer, whether elastomeric or not, with the peroxide cross-linking agent, involves the reaction of the peroxide with the polymer in its finished, fabricated state. Since this method involves the penetration of the polymer by the reactant pe o ides. t is a aptabl p im ly o ve y th nsections of polymer, ,such as in coatings rin unsupported The coating or film is maintained'in contact with a mixture of the peroxy-type compound and the basic metal compound, at elevated'temperaturesandpreferablyunder pressure, for a period of't'ime ranging from a few seconds llhis resultsin 'the changing .of :t'he

characteristics of the chain polymer to-those iof a space" polymer.

to several days.

The proportions of peroxyetype compound and basic metal compound used in the cross-linking ofthe highly fiuorinated polymers of this invention will -.vary with the nature of ;the polymer, the nature of the peroxy=type compound, the-'natureof the basic metalcompound, qthe method, time and temperature of curing and thedegreeof crosselinkingfiesired. r-I n general, for-each 100 parts iby weight of highly ifiuorinatedjpolymer, the weight of peroxy-typecompound will vary from about 0:5-to about will vary from about 1 to about 20 parts by weight.

iThetemperature andttimeof curing will also .vary with the .-nature.of zthe polymerzto becured, the nature -;of 'rthe peroxy-type compound .and the basic metal type compound and the degree .of cross-linking desired. Ingeneral, the temperature of .curing may vary from about 75 C.zto about 200 .C. The itirne of curing may vary from about 5 minutes to about 16 hours.

EXAMPLE -I 100 parts by Weight of an equimolar chlorotrifluoroefllylene vinylidene,fluoride 'copolymer were handed on .a

cooled;mill. added and blended into the rubbery copolymer. parts by weight .of benzqyl peroxide was .ground 5 parts by weight of .zinc .oxide was .then

to a fine powderand dispersed into the rubbery matrix on the .mill. The blend .Was then milled thoroughly and sheeted for pressing. A sheet -of this blend was press cured in a .standard.ru bber. moldmeasuring 6" square ;by 0.078 inch deep at 20,0.pounds per square inch and .C. for one-half hour. The pressxcured sheet was then cured in an ovenat .150" aC.,for..onehour. The cured sheet was :white in color and had smooth texture witha glossy surface. It .was .completelytree of bubbles or new lines.

Physical data :100 parts by:weight of an ;equimolar chlorotrifluoro-.

ethylene-vinylidene fluoridecopolymer was blended by a proceduremimilar .to that of Example I, with 5 parts by weight of zinc oxide and 3.2 parts *by weightof benzoyl peroxide. A sheet was press cured LfOl one-quarter hour at C. and oven cured for two hours at C.

Physical data Stress at Tensile Percent Shore Treatment 300% E Strength Elonga- Hard- (p. s. i.) (p. s. 1.) tion mass A (5 sec.)

Press cure plus oven cure 350 770 500 45 EXAMPLES III TO VII Additional blends, in accordance with the following The amount of basic'metal'compound 7 recipes, were made in accordance with the foregoing procedureto produce results as outlined in the table below:

11. The method of claim 9 wherein the organic peroxide is p-chlorodibenzoyl peroxide.

Example No 1. III IV V VI VII Recipe N 61 74 88 100 101 Parts by weight of:

E uimolar OF=OFG1IOF1= Ha copolymer 100 100 100 100 100 Z110 1O Ca 10 PbO l0 MgO 10 Sodium methylate C Benzoyl peroxide 3 3 3 3 3 are:

Press, hr./ O /11 4/110 110 36/110 %/110 Oven, hr./ O 16/149 16/149 16/149 16/149 16/149 Physical Prop;

Stress 300% E (p. s. i.).-. 530 490 380 615 330 Tensile strength (p. s. i.) 1, 350 1, 740 1, 425 1, 605 775 Percent Elongation 600 475 790 560 700 Shore A Hardness 47 49 50 52 49 We cla1m: 2O 12. The method of claun 8 wherein the peroxy com- 1. A method of cross-linking a'chain saturated polymer, containing fluorine substitution at at least half the positions available for such substitution and composed substantially exclusively'of carbon, hydrogen and halogen atoms which comprises heating said polymer in the presence of an organic peroxy compound which is relatively stable at temperatures below about 50 C. and in the presence of a basic metal compound.

2. The method of claim 1 wherein the chain polymer is a copolymer of a mono-olefinic compound containing a high degree of fluorine substitution and a mono-olefinic compound containing hydrogen, both compounds containing no atoms other than carbon, hydrogen and halogen atoms.

3. The method of claim 2 wherein the hydrogen containing mono-olefinic compounds contains CH groups.

4. The method of claim 2 wherein the hydrogen containing mono-olefinic compounds is vinylidene fluoride.

5. The method of claim 2 wherein the mono-olefinic compound containing a high degree of fluoride substitution is chlorotrifluoroethylene.

6. A method of vulcanizing a chain saturated elastomer, containing fluorine substitution at at least half the positions available for such substitution and composed substantially exclusively of carbon, hydrogen and halogen atoms, at least of the carbon atoms being bonded solely to hydrogen atoms and other carbon atoms, which comprises heating said elastomer in the presence of an organic peroxy compound which is relatively stable at temperatures below about 50 C. and in the presence of a basic metal oxide.

7. The method of claim 6 wherein at least about 10% of the carbon atomsare in -CH groups.

8. The method of claim 7 wherein the chain saturated elastomer is a copolymer of vinylidene fluoride and chlorotrifluoroethylene.

9. The method of claim 8 wherein the peroxy compound is an organic peroxide.

10. Themethod of claim 9 wherein the organic peroxide is benzoyl peroxide.

pound is ditertiarybutyl perbenzoate.

13. The method of claim 8 wherein the basic compound is zinc oxide.

14. The method of claim 8 wherein the basic compound is magnesium oxide.

15. The method of claim 8 wherein the basic compound is lead oxide (PbO).

16. The method of claim 8 wherein the basic compound is sodium methylate.

17. A method of preparing a cross-linked film of a highly halogenated polymer which comprises dissolving in a suitable solvent a chain saturated polymer, containing fluorine substitution at at least half the positions available for such substitution and composed substantially exclusively of carbon, hydrogen and halogen atoms andan organic peroxide, slurrying into the solution be References Cited in the file of this patent UNITED STATES PATENTS 2,157,997 Brous May 9, 1939 2,446,984 Rogers Aug. 10, 1948 2,495,286 Brubaker Jan. 24, 1950 2,628,214 Pinkney et al. Feb. 10, 1953 FOREIGN PATENTS 496,279 Canada Sept. 22, 1953 OTHER REFERENCES Polyethylene (Raff et al.). Published by Interscience Publishers Inc. (1956), New York, p. 137. 

1. A METHOD OF CROSS-LINKING A CHAIN SATURATED POLYMER, CONTAINING FLUORINE SUBSTITUTION AT AT LEAST HALF THE POSITIONS AVAILABLE FOR SUCH SUBSTITUTION AND COMPOSED SUBSTANTIALLY EXCLUSIVELY OF CARBON, HYDROGEN AND HALOGEN ATOMS WHICH COMPRISES HEATING SAID POLYMER IN THE PRESENCE OF AN ORGANIC PEROXY COMPOUND WHICH IS RELATIVELY STABLE AT TEMPERATURES BELOW ABOUT 50*C. AND IN THE PRESENCE OF A BASIC METAL COMPOUND. 